It is well known that Rotating Polarizer and Rotating Analyzer Ellipsometer systems can not accurately monitor DELTA values near 0.0 and 180 degrees, and Modulation Element Ellipsometers also have problems monitoring PSI values near 45 degrees. U.S. Pat. Nos. 5,757,494 and 5,956,145 to Green et al., which are incorporated herein by reference, teach the application of Berek-type Variable Retarders in such Ellipsometers to improve their data acquisition capability in the identified problematic angle regions. The System disclosed in said 494 and 145 Patents provides for tipping of a Variable Retarder about each of two orthogonal axes which form a plane which is substantially perpendicular to a beam of electromagnetic radiation which passes therethrough. The preferred approach is to acquire data with said Variable Retarder oriented at “0.0 Degrees and Tipped at + and at −45 degrees Rotations about said two orthogonal axes, and the resulting Five data sets are simultaneously applied in a common regression procedure. This Methodology has been found to greatly enhance the data Acquisition capability of Rotating Analyzer and Rotating Polarizer and Modulation Element Ellipsometers Systems in the identified problematic angle regions.
The Variable Retarder “Tip” Motion Implementing System disclosed in the 494 and 145 Patents, while performing well, has been found to be expensive and cumbersome to utilize. Said “Tip” producing system requires two separate rotation producing means which are oriented orthogonally with respect to one another. The present invention is therefore a new Retarder “Tip” producing system which enables practicing the Methodology of the 494 and 145 Patents, which are incorporated by reference into this disclosure.
In light of the approach to providing the equivalent to the five data sets identified above, Applicant also discloses knowledge of a Patent to Luecke, U.S. Pat. No. 5,245,478. Said 478 Patent describes a mount for Berek-type retarders which can be both tilted, and rotated via adjustment rings which are concentric to the optical path of the device, and within which a birefringent crystal is positioned. The present invention operates on a similar basis, but does not involve the use of concentric rings or place a variable retarder plate within anything like concentric rings.
Even in view of the prior art, need remains for an improved mounting system for Berek-type retarders which find application in ellipsometers.